The present invention is generally directed to a new and improved fire or security detection system. The present invention is more particularly directed to a communication line monitoring system for use in high security fire or security detection systems.
There are many applications in industry and commerce for fire and security detection systems. Such systems are employed, for example, in banks, industrial factories, or industrial research and engineering facilities.
Such systems generally include a plurality of remote fire or security sensors, such as motion detectors or normally closed switch contacts, used at places of access such as doors or windows. Such remote sensors are customarily coupled to a communication line which is also coupled to a central alarm station. The central alarm station monitors the condition of the communication line and issues an alarm indication when the condition of the communication line indicates that a fire has occurred or that a secured access point has been breached.
In high security systems, at the remote end of the communication line is a remote transmitter having a preassigned address which continuously transmits back to a monitor at the central alarm station a series of coded electrical signals corresponding to the preassigned address of the remote transmitter. The monitor is arranged to identify the preassigned address of the remote transmitter and, if an alarm condition associated with a communication line exists, the code or address received by the monitor will be different than expected. This can result for example, by a pair of normally open switch contacts associated with a given sensor shorting the communication line or by a pair of normally closed switch contacts opening the communication line.
In the past, the number of different codes or addresses which could be preassigned to a given remote transmitter has been limited because a binary transmission code has been used. Also, it is necessary for the monitor to be conditioned to identify the code of its associated remote transmitter. In the past, such monitors have been unalterably conditioned requiring replacement of the monitor when a remote sensor has been replaced with one having a different address.
The communication line customarily employed is a line pair having opposite polarity. Polarity reversal of the line pair has been used to switch the system from what is known as a secure state to what is known as an access state and vice versa. In the secure state, all remote sensors are activated. In the access state, customarily used during the day, some remote sensors can be deactivated by the polarity reversal to provide limited access to a secured area without provoking an alarm condition. For example, a sensor having a pair of normally closed contacts can have a diode coupled in parallel with it which is back biased in the secure state, thus activating the sensor, and forward biased by the polarity reversal in the access state, thus deactivating the sensor. In highly secure systems, it would be desirable for the central alarm station to be able to verify that the communication line is in the proper operating state.
Accordingly, it is a general object of the present invention to provide a new and improved fire or security detection system of the type including a central alarm station and at least one remote transmitter.
It is further general object of the present invention to provide a communication line monitor for a fire and security detection system which has a greater number of preassignable addresses for the remote transmitters.
It is a further object of the present invention to provide a communication line monitor for use in a fire or security detection system wherein the monitor, located at the central alarm station is resetable to any one of the other possible preassignable remote transmitter addresses or codes should a remote transmitter require replacement.
It is still a further object of the present invention to provide a communication line monitor wherein the secure or access state of the communication line can be confirmed by signals transmitted by the remote transmitter to the monitor of the central alarm station.